Analysis of Possible DHCPv6 and CGA Interactions
draft-ietf-csi-dhcpv6-cga-ps-09

This is a good document, even if the actual message is quite short
and simple, and at times the draft struggles a bit to explain its
conclusions clearly. FWIW, I disagree with some of the discusses that
I have seen. I do have a few concerns (one shared with Russ), however:

Section 3 claims without good justification that CGA parameters should
be managed by a central entity. I think a reasonable case can be made
for some parameters (like algorithms, Sec) but maybe not all. I can
see also good reasons why central administration would be a bad idea
(e.g., prefix is a local network matter and comes from the RAs). In any
case, the document is silent about all this.

Section 3 also suggests that with Sec>0 the generation of the hashes
is something that should be centralized. Again, I can see also some
reasons not to. In particular, Sec was not designed as a way to burden
current machines but to enable higher cost searches when machines get
faster. That is, if generating a Sec>0 value is a big burden for our
computers, we should probably still use Sec=0. (I think there is a better
argument than the one used in the draft. A very low-power host might
want to delegate its key and hash generation to a more general purpose
computer.)

Section 4 should expand on what the implications for using CGAs by
DHCPv6 servers. I believe that might actually be useful, as you could
tie different transactions with the same DHCPv6 server together, ensuring
that it is still the same node. But you could not prevent someone
claiming to be a DHCPv6 server.

I agree with other ADs that the draft should handwave less when it
describes the security implications of some new design. The idea of
the draft should be to describe what CGA can bring to DHCP security,
and the security implications cannot be skipped. One example where
you could easily write some more specific analysis is the above issue
on using unauthorized CGAs by DHCPv6 servers and what the security
impacts of that are.

Support DISCUSS positions from security ADs

  The direction suggested by this document will undermine the privacy
  features associated with host-generated CGAs.  In general, CGAs are
  not used in the same environment as a DHCP server, and I do not see
  a justification for this to change.

  Without providing a reason, this document asserts that local a
  administrator ought to manage CGA generation parameters.  I am
  guessing that the concern is the computation burden, but this is not
  compelling.  Further, RFC 3315 already allows a DHCPv6 server to
  reject a CGA generated with unacceptable parameters.

  This document discusses the use of DHCPv6 to assign certificates to
  CGA address owners.  CGA 'ownership' can already be validated with
  the private key, so the need for a certificate is unclear.

  This document states: "... the generation of the Modifier field of a
  CGA address is computationally intensive."  RFC 3972 seems indicate
  otherwise for most key sizes.  In general, RSA key pair generation is
  not a big concern on modern processors.  It might be a mild concern
  on mobile devices, but some detailed explanation is required.

This is an updated position based on -07.

1) I share the concerns raised by others.

2) s1:  Using CGA to protect DHCPv6 is 
   recommended though the concrete protocol is not defined in this 
   document.

In s2 it say: also using the CGA for DHCPv6 security purpose, analyzed 
   in section 4 this document, etc.

I didn't think this document was providing recommendations on analyzing possible solutions.  Maybe just delete this sentence?

3) s4: I'll have to think some more about this one:

  The usage of CGAs can also avoid DHCPv6's dependence on 
  IPsec [RFC3315] in relay scenarios.

How do CGAs provide replay protection?

4) s4: contains the following about pre-configured a CGA-enabled DHCPv6 server:

 But in this case the 
 address will be fixed. It may increase the vulnerability to, e.g., 
 brute force attacks.

Brute force attacks against?

5) I think this is a typo but I want to be sure:

 However, DHCPv6 servers are suitable to serve such computational 
 delegation requests from thousands clients. 

Shouldn't this be "not suitable"

6) s7 contains the following:

   Without other pre-configured security mechanism, like pre-notified 
   DHCPv6 server address, using host-based CGA by DHCPv6 servers could 
   not prevent attacks claiming to be a DHCPv6 server. 

I think you're talking about spoofing here.  Is this really true?  What happens when they use IPsec?

7) s7: One thing I think you also need to talk about is when the DHCPv6 server says no to the address based on a Sec value and then the DHCPv6 server says use a value one that is weaker.

So since the authors clarified in -09 that this is not a problem statement 
(seems late in the day but whatever) I re-read the document and re-wrote
my discuss points. However, the overall thrust of the discuss remains: I
don't see that this document is useful or needed.

But to the details:

#0 I agree with other discussing ADs.

#1 CGA's have privacy benefits, and the idea of "registering" those with a dhcp
server would entirely remove that feature.  I fail to see why a client would
*ever* want to register its CGA, and the document makes only one incidental
mention of privacy. That seems entirely insufficient to me. This echoes Russ'
discuss with which I fully agree. You could resolve this discuss point by for
example stating that, for privacy preserving reasons, there is no known
use-case where it would make sense for a DHCP client to use a CGA. Additional
changes would flow from that I would expect, if you were happy to take that
approach. (But then that leaves *very* little text.)

#2 Related to #1, but I don't get this bit:

  "However, after CGA has been defined, as an independent security
   property, many other CGA usages have been proposed and defined, such
   as SHIM6 [RFC5533], Enhanced Route Optimization for Mobile IPv6
   [RFC4866], etc. In these scenarios, CGAs may be used in DHCPv6-
   managed networks."

*Exactly* what use-cases exist for this involving a DHCP client?  You need to
explain, if you don't accept the approach suggested in #1.

#3 I agree that it might, in principle, be interesting to use CGAs for DHCP
servers or relays. However, I fail to see why section 4 (with DHCP client text
removed) justifies this document and only section 4 (<1 page) seems to 
remain, if you accept #1. There are sometimes reasons why an RFC might
be worthwhile with only one page of real content but this is not one of
those cases IMO. You can just include that page in the relevant DHC WG 
document.
 
I'm sorry, but I just don't think this is a useful document to produce.

I am submitting these issues as a discuss position, but please note that I expect to move from Discuss to Abstain
since I have significant issues with the overall direction.  My issues include one "discuss-discuss" and a number
of specific issues.

Discuss-discuss:
The document implies that centrally managed CGAs were envisioned in the original specs ("The current CGA
specifications do not mandate which device generates a CGA address.")  However, I see no evidence in RFC 3972
that centrally generated CGAs were envisioned.  My reading assumes locally generated CGAs.  Is there any
evidence that the wg really thought CGAs would be generated centrally?

Specific Issues:

In my opinion, this document fails to establish that a problem exists. The current DHCPv6-CGA coexistence model
is dismissed without a clear explanation, and the impact of centrally generated key pairs and modifiers on the
security assumptions for protocols that employ CGAs is not explored at all. [sections 1 and 2]

I believe that sections 3 and 4 should be addressed in separate documents, unless the authors believe that DHCPv6
only benefits from centrally managed CGAs.  Putting both concepts in one document is confusing. [global]

Increasing the security of a CGA against a brute force attack while weakening the base security assumptions
may not be a good compromise.  [section 3, glaringly absent from section 5]

The rationale for centrally generated key pairs is very weak, since a node (e.g. a cell phone) can reuse the same
key pair each time it joins a subnet and generates a new CGA.  [section 3]

It is unclear whether the concepts proposed in Section 4 ("What CGA can do for DHCPv6") rely on centrally 
managed CGAs or not.  From what I can tell, traditional CGAs might be very useful for DHCPv6 deployments,
but I am unsure about that.

The document states that when DHCPv6 is used to manage CGAs "it does not increase privacy risks".  Relative
to what?  IMHO, centrally generated CGAs have to increase privacy risks in comparison to CGAs generated by the
host itself. [section 5]

As stated in the security consideration, "This work does not include a complete security analysis".  In my opinion, 
such an analysis is crucial to determining whether this "problem" should be solved. As alluded to above, that
analysis needs to review the impact on current protocols that use CGAs.  If those protocols assume local
generation, what is the impact of the mechanisms described here? [section 5]